1. Field of the Invention
The present invention relates to a speaker apparatus of a digital driving method.
2. Description of the Related Art
Speaker apparatuses are generally driven in accordance with an analog audio signal. Speaker apparatuses which can be driven directly in accordance with a digital audio signal have been conceived.
In FIG. 3, reference numeral 10 denotes an example of a speaker unit for use in such a speaker apparatus of a digital driving method. This speaker unit 10 is constructed into an electromagnetic coupling type. In this example, a cutout 13 is formed around the front end portions of a center pole section 12 of a yoke 11 formed of the center pole section 12 and a flange section 14, and a primary coil 1 which will be described later is mounted to this cutout 13.
In this case, the primary coil 1 is mounted in such a manner that it is wound as an air-core coil and bonded to the cutout 13, or it is directly wound around the cutout 13, or although not shown, it is wound around a bobbin made of a magnetic material and the bobbin is fitted into the cutout 13 and bonded thereto.
Further, in the flange section 14 of the yoke 11, a hole or opening 15 is formed in a part of a position proximate continuously to the center pole section 12, with a terminal plate 16 mounted on the rear surface of the flange section 14. Then, a lead wire 17 of the primary coil 1 is bonded onto the peripheral surface of the center pole section 12 and inserted into the opening 15, and is connected to a terminal 18 of the terminal plate 16 by soldering or the like.
Further, an annular permanent magnet 21 is bonded onto the front surface of the flange section 14, an annular plate 22 is bonded to the front surface of the permanent magnet 21, and thus a magnetic circuit 20 having a gap 23 between the outer peripheral surface of the front end portion of the center pole section 12 and the inner peripheral surface of the annular plate 22 is formed.
Further, a secondary coil 2 is disposed in the gap 23. In this example, this secondary coil 2 is formed as a cylindrical body made of a non-magnetic conducting material, for example, aluminum and formed into a short coil of one turn.
Further, the inner peripheral portion of a vibration plate, i.e., cone 32, a dust-tight cap 33, and the inner peripheral portion of a damper 34 are mounted to the secondary coil 2, and a speaker frame 35 is mounted to the plate 22. An edge 31 is mounted to the outer peripheral portion of the cone 32, the edge 31 and a gasket 36 are mounted to the speaker frame 35, and the outer peripheral portion of the damper 34 is mounted to the speaker frame 35.
If it is assumed that the digital audio signal is in a format used for CDs and DATs, that is, the number of quantization bits is 16 bits and in the form of two's complement, as shown in FIG. 5, the MSB thereof is a sign bit, which indicates the polarity of the analog audio signal when this digital audio signal is converted from digital to analog form, and 2SB to LSB of which indicate the level of the analog audio signal.
Thus, the above-described primary coil 1 is formed of, for example, 15 coils 1A to 1N, and 1P in correspondence with the digital audio signal, as shown in the lower part of FIG. 5. The number of windings of the coils 1A to 1P is made to be a value corresponding to the weight of each bit of the digital audio signal.
More specifically, as shown in, for example, FIG. 5, if the coils 1A to 1P correspond to the LSB to 2SB of the digital audio signal, the number of windings of a coil corresponding to a certain bit is made twice the number of windings of a coil corresponding to the bit of one lower order, for example, the number of windings of coil 1A is 2, the number of windings of coil 1B is 4, the number of windings of coil 1C is 8, and so on. That is, the ratio of the windings of the coils 1A to 1P is 2**0 to 2**14 (x**y indicates the y-th power of x in the geometrical series of common ratio 2. Hereinafter the same applies).
FIG. 4 shows an example of a signal system of the speaker apparatus of a digital driving method. Reference numerals 44A to 44N, and 44P denote constant-current circuits. These constant-current circuits 44A to 44P are connected to positive and negative power lines and operate in a forward-type mode or reverse-type mode in accordance with a control signal from an external source. In the forward-type mode, a DC current (constant current) I of a predetermined fixed magnitude flows to a load, and in the reverse-type mode, a DC current I of the same magnitude as that in the forward-type mode flows from the load.
Bidirectional switching circuits 45A to 45N, and 45P, and the coils 1A to 1N, and 1P of the speaker unit 10 are connected in series between the output terminals of the constant-current circuits 44A to 44N, and 44P and a ground, respectively.
Further, a digital audio signal SD reproduced from, for example, a CD is supplied to a shift register 42 with serial input and parallel output through an input terminal 41. The signal SD is converted into parallel data for each sampling, the MSB of the converted signal SD is supplied as a switching signal between the forward-type mode and the reverse-type mode to the constant-current circuits 44A to 44P, controlling the constant-current circuits 44A to 44P so as to operate in the forward-type mode when MSB="0" and to operate in the reverse-type mode when MSB="1".
Further, the LSB to 2SB of the signal SD from the register 42 is supplied to a decoder circuit 43 and the MSB is supplied to the decoder circuit 43. Thus, the LSB to 2SB of the signal SD are converted or decoded into the form of lower-order 15 bits in a reflected binary code, namely, binary data indicating the absolute value of the signal SD, as shown in FIG. 6.
Then, the LSB to 2SB after conversion are supplied to the switching circuits 45A to 45P as control signals; when the bit is "0", the corresponding switching circuit is turned off, and when the bit is "1", the corresponding switching circuit is turned on.
With such a construction, when current I flows through the primary coil 1 (1A to 1P), since the primary coil 1 and the secondary coil 2 are electromagnetically coupled to each other, an electric current is induced in the secondary coil 2, and this current flows through the secondary coil 2. Therefore, similarly to a conventional speaker, the cone 32 deviates in a forward or backward direction in correspondence with the polarity of the electric current which flows through the primary coil 1.
When the MSB of the digital audio signal SD is "0", the constant-current circuits 44A to 44P are set to a forward-type mode operation. Therefore, the constant currents I to I flow through components in the following order: the constant-current circuits 44A to 44P, the switching circuits 45A to 45P, the primary coils 1A to 1P, and the ground. When the MSB of the signal SD is "1", the constant-current circuits 44A to 44P is set to the reverse-type mode operation. Therefore, the constant currents I to I flow through components in the following order: the ground, the primary coils 1A to 1P, the switching circuits 45A to 45P, and the constant-current circuits 44A to 44P.
That is, the polarity of the constant currents I to I which flow through the coils 1A to 1P reverses according to the value of the MSB. Therefore, the deviation direction of the cone 32 of the speaker unit 10 is controlled by the MSB of the signal SD.
Further, the LSB to 2SB of the signal SD output from the decoder circuit 43 indicate the absolute value of the analog audio signal when the signal is converted from the signal SD from digital to analog form. When a certain bit of the LSB to 2SB is "1", the corresponding switching circuit of the switching circuits 45A to 45P is turned on, causing a constant current I to flow through a corresponding primary coil of the primary coils 1A to 1P.
At this time, since, for example, the number of windings of the coil 1B is made twice the number of windings of the coil 1A, the magnitude of the electric current which flows through the secondary coil 2 when the current I flows through the coil 1B becomes twice the electric current which flows through the secondary coil 2 when the current I flows through the coil 1A. The same applies for the other adjacent coils.
That is, even if the magnitude of the constant currents I to I which flow through the coils 1A to 1P is equal, since the ratio of the number of windings is set as described above, the magnitude of the electric current which flows through the secondary coil 2 differs in correspondence with the ratio of the number of windings. The ratio of the number of windings of the coils 1A to 1P is a value corresponding to the weight of each bit of the digital audio signal SD. Therefore, when the cone of the speaker unit 10 deviates, the amount of the deviation corresponds to the absolute value indicated by the LSB to 2SB.
As a result of the above, the cone 32 of the speaker unit 10 deviates in a direction and by an amount corresponding to the MSB and the LSB to 2SB of the digital audio signal SD for each sampling. Therefore, a reproduction sound of the digital audio signal SD is output from the speaker unit 10.
In this case, the digital audio signal has been digitized at a sampling frequency of 44.1 kHz or 48 kHz. Since the coils 1A to 1P are driven in accordance with the digital signal, the low-frequency components of the analog audio signal before being digitized become a high frequency over 20 kHz as a signal current flowing through each of the coils 1A to 1P of the primary coil 1. Therefore, the speaker unit 10 is able to reproduce low-frequency components.
Similarly to a conventional speaker, the vibration system of the speaker unit 10 has difficulty responding to high frequencies and can hardly reproduce high-frequency components, in particular, components over 20 kHz. Therefore, even if the primary coils 1A to 1P are driven in accordance with the digital audio signal of a sampling frequency of 44.1 kHz or 48 kHz, the components of the sampling frequency are hardly reproduced. If the components were reproduced, since they are reproduced with very small sound pressure and sound with a frequency over 20 kHz can hardly be heard by the human ear, no inconvenience occurs.
Thus, according to this speaker apparatus, it is possible to convert a digital audio signal into a reproduction sound without performing D/A conversion.
However, in the above-described speaker apparatus of a digital driving method, the constant-current circuits 44A to 44P must operate in the forward-type mode or the reverse-type mode according to the MSB of the signal SD. Such constant-current circuits become complex in construction, resulting in a high cost.
Further, when the output of the speaker apparatus is increased, the current I which flows through the coils 1A to 1P must be increased; therefore, the construction becomes more complex and expensive. Moreover, as many as 15 such constant-current circuits are required as indicated by the reference numerals 44A to 44P in FIG. 4.
Furthermore, since the constant currents I to I to be supplied to the coils 1A to 1P are turned on and off respectively according to "0" and "1" of the LSB to 2SB of each sample of the digital audio signal SD, the highest frequency thereof becomes the sampling frequency of 44.1 kHz or 48 kHz of the signal SD. For this reason, the impedance when the coils 1A to 1P are seen from the constant currents I to I becomes several k.OMEGA., and the switching circuits 45A to 45P must supply the constant currents I to I to the coils 1A to 1P having such a large impedance. Likewise, the construction becomes complex and expensive.